Mechanical reduction gears transmitting high outputs are lubricated with oil under pressure by means of one or more lubrication pumps driven by gear trains moving on one main chain of the reduction gear or by an annexed drive source. These reduction gear devices need to be cooled intensely as any internal friction produces significant heating of the lubrication oil which, if it becomes excessive, diminishes the lubricating power of the heated or disintegrated oil, thus compromising the reliability of the devices and resulting in a falling off of the mechanical characteristics of materials subjected to high temperatures.
So as to ensure this cooling, an air/oil exchanger is generally used, the oil circulating under pressure inside that exchanger. On land or aeronautical vehicles, heat exchange is effected with air blown through a radiator, either by a ramming intake if the vehicle continuously has sufficient speed, or by a fan when the output to be evacuated is high and the vehicle has no actual speed during certain usage phases, this applying when a helicopter is hovering.
With helicopters subjected to slightly less high outputs, such as less than 600 Kw, the reduction gear members may be cooled without without any exchanger, provided the exchange surfaces of the housing of the reduction gear are artificially increased and are blown so that the temperature of the oil does not exceed a threshold adversely affecting the reliability of the members or which in any event do not reach a limit beyond which they would deteriorate (about 150.degree. C.). The absence of an exchanger is clearly an advantage as it enables costs to be significantly reduced and avoids the use of the corresponding pipes and connectors, these items of equipment being extremely vulnerable and which may result in a total loss of oil should a leak or breakage occur, the pipes and connectors needing to be laid as quickly as possible.
On the other hand, for the main gearboxes of higher-powered helicopters, for example with an output of 1000 Kw upwards, it is necessary to use one or several exchangers ventilated by one or several fans. These exchangers and their cooling circuits do, however, result in taking on additional weight which, by way of example, may be 30 Kg for a bimotor main gearbox of 2000 Kw whose intake speeds are those of turbomotors (exceeding 20,000 rmp). Apart from the fact that this additional weight is difficult to accomodate and that the exchangers in question generate risks of the oil leaking, they do introduce a metallic intermediary which complicates and slows down exchangers at the expense of their effectiveness.